Functionalized polymers can be prepared by anionic polymerization methods, e.g., by initiating polymerization of 1,3-butadiene with a functionalized initiator and/or by reacting a living anionic polymer with a functionalizing agent. The functionalized polymers made by anionic polymerization methods can have a high functionality, resulting in polymers that have a greater affinity toward carbon black or silica fillers than non-functionalized polymers. Therefore, the rubber vulcanizates made from the functionalized polymers give lower hysteresis loss than those made from non-functionalized polymers. Unfortunately, stereoregular polymers such as cis-1,4-polydienes, which are often required for the manufacture of certain tire components, cannot be obtained by anionic polymerization methods because these methods do not provide strict control over the polymer microstructure such as the cis-1,4-linkage content of a polydiene polymer.
Coordination catalysts (also known as Ziegler-Natta catalysts), such as lanthanide-based catalysts comprising a lanthanide compound, an alkylating agent, and a halogen-containing compound, are often highly stereoselective. These catalysts can produce conjugated diene polymers having high cis-1,4-linkage contents. The resulting cis-1,4-polydienes are particularly suitable for use in tire components such as sidewalls and treads. Nevertheless, the use of coordination catalysts in polymerization limits the ability to functionalize the resulting polymers because coordination catalysts operate by rather complex chemical mechanisms that involve the interaction among several catalyst components and often involve self-termination reactions. As a result, it is difficult to prepare highly functionalized polymers under ordinary conditions by using coordination catalysts.
Cis-1,4-polydienes that are prepared via solution polymerization processes catalyzed with lanthanide-based catalysts are known to display some degree of pseudo-living characteristics, such that some of the polymer chains possess reactive chain ends. Accordingly, these cis-1,4-polydienes may react with certain functionalizing agents, such as aminoketones, heterocumulene compounds, three-membered heterocyclic compounds, organometallic halides, and certain other halogen-containing compounds. Unfortunately, due to the above-mentioned limitations associated with a coordination catalyst, the resulting functionalized cis-1,4-polydienes generally have a lower functionality as compared to other functionalized polymers that are produced by anionic polymerization methods.
Therefore, there is a need to develop a method for producing functionalized cis-1,4-polydienes that have a combination of a high cis-1,4-linkage content and a high functionality.